A boundary acoustic wave device has a structure in which an interdigital transducer (IDT) is disposed at the interface between different media. In such a boundary acoustic wave device, boundary acoustic waves propagate in a laminate prepared by laminating the different media. Accordingly, in the boundary acoustic wave device, a complex package structure can be omitted. Therefore, the structure of the boundary acoustic wave device can be simplified and the thickness thereof can be reduced, as compared with a surface acoustic wave device.
In the boundary acoustic wave device, when the operating frequency is increased, the period of an IDT is decreased. Therefore, the width of electrode fingers constituting the IDT and reflectors is decreased. Consequently, the conductor resistance of the electrode fingers is increased, thereby increasing the loss.
On the other hand, in the boundary acoustic wave device, when the sound velocity of boundary acoustic waves is controlled to be lower than the sound velocity of transverse waves propagating through the media disposed on and under the interface, the boundary acoustic waves are confined between the upper and lower media. Accordingly, the propagation loss can be decreased.
In order to increase such a confinement effect, it is effective that the IDT is formed using a metal having a high density. Hitherto, as described in Patent Document 1, the IDT of boundary acoustic wave devices is often composed of Al. In contrast, in the boundary acoustic wave device described in Patent Document 2, in addition to Al, Au and Ag are also disclosed as the IDT material.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 58-30217    Patent Document 2: DE4132309A1